1. Field of the Invention
This invention relates to lasers and more particularly to organic dye lasers. Even more particularly, this invention relates to oxazole dyes and certain quaternary salts of those compounds used in solution with non-interferring polar solvents to form lasing media useful in organic dye lasers
2. Description of the Prior Art
Many laser dyes lase effectively in organic solvents but become poor or ineffective in water or organic solvents containing large amounts of water even though the dye itself is water soluble. Water is a preferred solvent because it is not flammable and because it has excellent thermooptical properties. Certain modifications of the molecular structure of the laser dye can produce somewhat predictable changes in lasing performance. However, it is extremely difficult to incorporate all of the desired properties into a single laser dye. One particular problem is that organic molecules in aqueous solution have a tendency to form dimers and higher aggregates which have a distinctly different absorption spectrum from the monomer. In most cases, the fluorescence of the dimers is completely quenched and cannot be observed. Adding to this problem is that the equilibrium between monomers and dimers shifts to the dimers with increasing dye concentration and with decreasing temperature. It is possible to suppress the aggregation of dyes in aqueous solutions by adding surfactants. However, because of the large amounts that are needed, the lasing properties of such solutions are not always favorable.
Certain oxazole dyes are well-known as stable, long-lived laser dyes that lase in water and aqueous solutions. The quaternary salts of 2-(4-pyridyl)-5-aryloxazoles are known to be useful as visible wavelength lasing dyes when used with non-interferring polar solvents such as low molecular weight alcohols and water. These dyes generally have good photochemical stability and lase in the 500-600 nm region. Previous investigations into laser dyes that lase in the blue-green region of the spectrum suggest some possible means of shifting the untuned lasing maximum to shorter wavelengths. Some methods discussed by L. Lee et al. in Water Soluble Blue-Green Lasing Dyes for Flashlamp-Pumped Dye Lasers, IEEE Journal of Quantum Electronics, vol. QE-16, no. 7 (July 1980) include decreasing the concentration of the dye, lowering the temperature of the dye solution, using in the 2-position of the oxazole ring a 3-pyridyl group rather than a 4-pyridyl group, and replacing the CH group in the oxazole ring with a nitrogen atom to give a 1,3,4-oxadiazole ring system. It was also suggested that improvements to output and stability could be made by adding or changing groups to the 4-position of the oxazole ring and /or the pyridinium nitrogen atom.
ln contrast, the present invention provides high-output long-lived dyes produced by other means. Halide groups and methoxy groups were added to the 4-position of the phenyl group at the 5-position of the oxazole ring. 2-(4-pyridyl)-5-(4-halophenyl) oxazoles and 2-(4-pyridyl)-5-(4-methoxyphenyl) oxazoles and their quaternary salts were prepared. Changes in the electronegativity of the group at the 4-position of the 5-phenyl ring were expected to affect the lasing wavelength. It was not expected that these changes would affect the lasing output or dye stability. The fluoro derivative was found to have good photochemical stability. Quite unexpectedly, this derivative was found to have a lasing output about 300% greater than the previous pyridyl phenyl oxazole dyes. It should be noted that the laser dyes of the present invention respond well to flashlamp pumping and the lasing data provided in Tables 1-3 resulted from dyes that were flashlamp pumped.